How many feet does a pound of seed plant?...

Firstly seed size is a consideration. Generally there are 10-15 seed pieces per pound

Types like Khing Yai or Big Kahuna will usually have less seeds per pound than other varietals but not always.

Smaller sized material will plant more feet per pound.

Some considerations for planting distances

The closer you plant seed pieces the more timely and often hilling will need to be. That is not a bad thing, just a consideration. When growing in the field commercially most growers in Hawaii plant even closer than we recommend and perform multiple hilling operations throughout the year. If you leave some space between seeds the plants have a little more time to grow horizontally before their irrepressible urge to grow upwards begins. Planting at a closer spacing also allows the row establish a canopy early on which will definitely help it shade the soil and developing rhizomes. So close spacing is a good choice in arid areas as hot soil temps can stunt the plants depending on frequency, duration and intensity of hot weather. Conversely far spacing of seed pieces may be an effective strategy when dealing with pests and disease pressure...In many countries this is the norm.